German published patent specification No. 3,225,228 describes passing a helically-grooved cable core through a unit for filling the grooves with a viscous filler material, such as a petroleum jelly, polyisobutylene, or a silicone jelly, then placing optical fibers on the open sides of the grooves, and then burying the fibers by passing the core through a part of smaller diameter including an escape orifice upstream therefrom for evacuating excess viscous material. The grooved core is then surrounded by a metal tube which is shaped by a funnel and then longitudinally welded.
Such a method does not enable the optical fibers to be placed in the grooves with a degree of slack to protect them against mechanical stresses to which the cable may be subjected while being handled and laid. A known manner of ensuring that optical fibers have such a desired degree of slack is to subject the core to tensile forces that elongate the core while the optical fibers are being inserted in the grooves. The tensile force is then removed, the cable core shortens and the fibers are then slackly received in the grooves. The tensile force or traction may be applied by means of a unit for stretching the cable between two caterpillar track arrangements. The tension disappears downstream from the second track arrangement. However, if the grooves are filled with viscous material upstream from a pulling track arrangement, there is a risk of the core slipping, and if the viscous material is inserted while hot, the mechanical properties of the core are also affected.
Preferred implementations of the present invention provide a method of injecting a viscous material during the manufacture of optical fiber cables which ensures that the grooves are rapidly filled with viscous material while the optical fibers are at a distance from the bottoms of the grooves, ie. while the fibers have the desired degree of slack since the core has left the tracked tensioning unit. Such implementations also prevent the viscous material from escaping upstream from its point of injection which would have the undesirable effect of urging the fibers backwards along their grooves and possibly damaging them.